Anthracene-PMDA

Stacks in which the neighbouring molecules are equidistant are termed regular, while those with dimerisation are termed alternating. The different stackings are of vital importance for determining the character of these materials as insulators, semiconductors, or metallic conductors. Fig. 2.17 indicates schematically that the TTF-TCNQ crystal is built up from separate stacks. An example of mixed stacks is the insulator anthracene-PMDA (Fig. 6.14). 

Fig. 6.16 Detection of a CT exciton in anthracene-PMDA by means of the Stark effect. The sharp zero-phonon line at the absorption edge (dashed line, without an electric field) splits in the presence of an electric field (ofca. 4 lO Vcm" ) into two components (solid line). From [38].

See in this connection the examples given in Chaps. 1 and 2, anthracene-TCNB (Fig. 1.6) and in Chap. 6, anthracene-PMDA (Fig. 6.14). 

The conductivities of these crystals lie typically in the range of 1(T to 10 Q cm clearly higher than those of pure anthracene crystals, for example. Their ground states, as shown by photoelectron spectra, are similar to anthracene, with 5-10% CT character. The triplet states Ti are likewise similar to anthracene, as seen in their optical spectra. The Si <- So absorption is, however, very different it is a typical CT absorption. As its photoconductivity shows, the charge separation in anthracene-PMDA takes place in the Si state with an activation energy of 0.14 eV. 

Fig. 6.14 The anthracene-pyromellitic acid dianhydride (A-PMDA) mixed-crystal system as an example of a weak donor-acceptor crystal with mixed stacks. The ground state is neutral, nearly without charge transfer. The lowest CT state Is formed by a transition of an electron...

In crystals which are composed of two different partner molecules, CT excitations and with them CT excitons are frequently the predominant lowest excitation states and are thus responsible for the lowest-energy transitions in the singlet system. We will illustrate this using the example of the weak donor-acceptor complex anthracene/pyromellitic acid dianhydride, (A/PMDA) (Fig. 6.14). The ground state is neutral and nonpolar, with only a small charge-transfer fraction. The lowest optical excitation starts from the ground state of the donor D, anthracene, (from its highest occupied orbital or HOMO) and leads to the lowest unoccupied orbital (LUMO) of the acceptors A, PMDA, within the mixed stack DADADA. A polar ex-... 

In this chapter, we will not discuss the weak CT complexes further. In these complexes, at least in the ground state, the charge transfer S is not large. They in general crystallise in mixed, alternating stacks. We have already mentioned the example systems anthracene-tetracyanobenzene or A-TCNB (cf Fig. 1.6) and anthracene-pyromellitic acid-dianydride or A-PMDA (cf Fig. 6.14). 

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